Find the lcm of the pair of numbers. 9 and 15

The LCM is 45. 

The prime factorization of 9 is 3 × 3.
The prime factorization of 15 is 3 x 5.

Multiply each factor the biggest number of times it occurs above (3 occurs twice in the prime factorization of 9, and 5 occurs once in the prime factorization of  15) to find the LCM.  3 × 3 × 5 = the LCM